1. Field of the Invention
The present invention relates to a technology for controlling the bias point of an optical transmitter automatically.
2. Description of the Related Art
A conventional optical transmitter for an optical communication system uses a modulation system, such as a direct modulation system, an internal modulation system, and an external modulation system, which is suitable for a type of a light source and a communication speed. In recent years, the external modulation system having a small variation in the wavelength of an optical signal (that is, a chirping) regardless of a transmission speed calls attention, and has been widely used.
FIG. 8 is a block diagram of a conventional optical transmitter using the external modulation system. As shown in FIG. 8, an optical transmitter 800 includes a light source 810, an external modulator 820, a branch unit 830, a photoelectric converter 840, a feedback unit 850, an oscillator 860, a pulse driver 870, a synchronous detector 880, and a bias controller 890.
The light source 810 generates unmodulated light by a light-emitting element, and outputs the unmodulated light to the external modulator 820. The external modulator 820 modulates the unmodulated light according to electric signals input from the pulse driver 870 and the bias controller 890, and outputs the modulated light to the branch unit 830 as an optical signal. The branch unit 830 branches the optical signal into two parts at a predetermined rate, for example, 9:1. The main part is input to an optical transmission path, while the other part is input to the photoelectric converter 840.
The photoelectric converter 840 converts the optical signal input from the branch unit 830 into an electric signal, and outputs the electric signal-to the feedback unit 850. The feedback unit 850 includes a filter 851 and an amplifier 852. Only the low-frequency component of the electric signal from the photoelectric converter 840 passes through the filter 851 to the amplifier 852, which amplifies the low-frequency component and outputs the amplified low-frequency component to the synchronous detector 880.
A reference low-frequency signal is generated by the oscillator 860, and output to the pulse driver 870 and the synchronous detector 880. Transmission data is input to the pulse driver 870 as an input signal, superimposed on the reference low-frequency signal from the oscillator 860, and output to the external modulator 820 as a modulation signal.
The synchronous detector 880 compares the low-frequency component of the electric signal from the amplifier 852 of the feedback unit 850 with the reference low-frequency signal from the oscillator 860, and outputs a signal corresponding to a phase difference to the bias controller 890. The bias controller 890 adjusts the potential of a bias signal (a bias point) to be input to the external modulator 820 based on the signal input from the synchronous detector 880.
As described above, a modulation signal from the pulse driver 870 and the bias signal from the bias controller 890 are input to the external modulator 820. The light transmission factor of the external modulator 820 varies according to the potential of the bias signal. The transmission factor is represented by an extinction characteristics curve specific to each kind of external modulators and each element of the external modulators. In other words, even when a modulation signal of the same amplitude is input, the amplitude of the optical signal output from the external modulator 820 varies greatly depending on the setting of the potential of the bias signal (the bias point). Therefore, in the block diagram of the optical transmitter 800, the optical signal is fed back to the external modulator 820 through the branch unit 830 to maximize the amplitude of the optical signal. Such a technique is disclosed in, for example, Japanese Patent Application Laid-Open No. H10-123471 and Japanese Patent No. 3333133.
However, according to the optical transmitters described in the above documents, the amplitude and the bias point need to be set while monitoring the waveform each time when an optical signal is transmitted, which is troublesome for the user.
Furthermore, the conventional optical transmitter cannot adjust the bias point appropriately when the extinction characteristics of the optical modulator change greatly according to the usage environment or the usage time. Therefore, the amplitude of the modulation signal needs to be set large to achieve the extinction ratio required for the optical transmission, thereby increasing the power consumption.